This article is for mountain bikes, not folding bikes. Don't use your folder on off-road trails.

Assuming you are on a trail that goes downhill, then sharply shifts uphill, essentially the trail forms a V shape. How should a rider deal with it? Riding it all the way down will undoubtedly lead to a crash, since the bike will run straight into the trail.

Ride the downhill portion as usual. Remove speed before you hit the bottom of the descent, and lift your front wheel so it starts on the ascent, while the rear is still on the downwards slope. Practice and you will be able to hold more speed in V's.